1. Field of the Invention
The present invention relates to a cue signal detection circuit mounted on a magnetic recording and reproducing device which reads and writes data from/on a magnetic tape for detecting, from among reproduction control signals recorded on the magnetic tape, a cue signal recorded at a predetermined cue position.
2. Description of the Related Art
A magnetic recording and reproducing device which reads and writes data from/on a magnetic tape such as a video equipment has a capstan motor mounted thereon for driving rotation for fast-forwarding and rewind of a magnetic tape. For controlling rotation of a capstan motor a cue signal detection circuit is provided which reads reproduction control signals recorded on a magnetic tape and detects a cue signal from among the signals. Reproduction control signal is a signal recorded, at the time of recording of a data signal on a magnetic tape, together with the data signal for the purpose of controlling reproduction of the data signal. Cue signal is a reproduction control signal recorded at a predetermined cue position (e.g. at a position of a data signal on a magnetic tape at the start of recording) for the purpose of indicating the cue position.
One example of conventional cue signal detection circuits of a magnetic recording and reproducing device of this kind is recited in Japanese Patent Laying-Open (Kokai) No. Hesei 5-135433, entitled "Cue Signal Detection Unit of Magnetic Recording and Reproducing Device". The cue signal detection circuit recited in the literature detects a cue signal based on a duty ratio of a reproduction control signal reproduced from a magnetic tape and in response to the detected cue signal, repeats rewind and fast-forwarding of the magnetic tape to locate the cue position. More specifically, every time a mode control circuit passes a recording position of a cue signal on the tape to detect the cue signal, fast-forwarding and rewind of the magnetic tape are repeated to gradually shorten a travel distance, thereby ultimately reaching the recording position of the cue signal (cue position). In addition, a pulse signal in accordance with a rotation speed of a capstan motor which is directly detected from the capstan motor by a capstan head (hereinafter referred to as a capstan signal CFG) is counted. The counted value corresponds to a travel distance of the magnetic tape and one pulse is equivalent to a travel distance of one centimeter. Then, based on a travel distance of the magnetic tape converted from the counted value of capstan signals CFG, a rotation speed of the capstan motor is controlled. Therefore, a time from when position of a cue signal is specified until when running is stopped is constant, independent of the rotation speed of the capstan motor.
One example of conventional techniques of improving a control signal duty ratio determining method aimed at reducing a time required for locating a reproduction control signal is recited in Instruction Manual for LSI MN67512/MN67520 manufactured by Matsushita Electric Works, Ltd. (published in January 1991, pp.128-145). FIG. 8 shows a relationship between a magnetic tape and a capstan motor in the magnetic recording and reproducing device recited in this Instruction Manual, while FIG. 9 shows structure of a cue signal detection circuit in a block diagram. With reference to FIGS. 8 and 9 together, the conventional cue signal detection circuit includes a CTL head 201, an amplification circuit 202, a timer unit 203, a high-period capture unit 204, a low-period capture unit 205 and a comparison circuit 206, with a magnetic tape 101 run at a high speed by a capstan motor 102 which conducts fast-forward and rewind of the magnetic tape 101 to take in a reproduction control signal CTL recorded on the magnetic tape 101 through the CTL head 201.
The reproduction control signal CTL taken in by the CTL head 201 is sent to the amplification circuit 202, amplified to have an appropriate signal level there and then output as a reproduction control signal CTL converted into a digital signal. The high-period capture unit 204 takes in a value of the timer unit 203 on a leading edge of the applied reproduction control signal CTL, as well as initializing the timer unit 203. The low-period capture unit 205 takes in a value of the timer 203 on a trailing edge of the reproduction control signal CTL. The comparison circuit 206 compares the values of the timer taken in by the high-period capture unit 204 and the low-period capture unit 205 to determine whether a cue signal exists or not based on the comparison results.
The above-described conventional cue signal detection circuit has a drawback that in cue signal detection made when a magnetic tape travels at a low speed and a high speed, limitations are imposed on a traveling speed of the magnetic tape.
The reason is as follows. In a conventional cue signal detection circuit, since a traveling speed and a frequency of a reproduction control signal CTL is proportional to each other, the higher a traveling speed of a magnetic tape is, the higher the frequency of a reproduction control signal CTL becomes, and the lower the traveling speed of the tape is, the lower the frequency of the reproduction control signal CTL becomes. In addition, an ordinary reproduction control signal CTL is recorded with a duty ratio of 60% and a cue signal is recorded with a duty ratio of 27.5%. Then, a pulse width is measured in every cycle of the reproduction control signal CTL because a counted value of capstan signals CFG which is obtained by the timer unit 203 is used for detecting a cue signal of a reproduction control signal CTL. Therefore, a frequency of a pulse enabling determination of a duty ratio of a reproduction control signal CTL is dependent on a reference clock CLK supplied to the timer unit 203, the number of CFG signals countable by the timer 203 and a duty ratio determination processing time.